text:
Is it possible to extend an AFSP training request?
My training request expires in a week or so. Is it possible to extend it or do I have to submit a new one if I'm running out of time?

It appears you're asking about TSA training which wouldn't be covered as part of aviation. You might get help at Travel.SE, as they address a lot of TSA issues, but your best bet would be to contact the office where you submitted your request and ask them.

@FreeMan I believe he's submitted a request for TSA approval to start flight training under the AFSP and he wants to extend the processing deadline. If so, it has nothing to do with air travel, although calling the AFSP helpline is probably still the best answer (this scenario isn't in their FAQ).

ah, I see, @Pondlife. Based on the previous VtC and the single TSA tag, I was attempting to be helpful. My misunderstanding.

@FreeMan No big deal :-) It made me think about a separate afsp tag, but we have very few TSA questions anyway so I don't think there's any big benefit to adding another tag right now

Assuming that you're in category 3 (aircraft under 12,500 lbs) then an approval is only valid for 365 days, so if you want to continue flight training after that then you need to request a new one:

Once you have permission to train, you have 180 days to begin training
  and 365 days to complete the approved training. Both of these time
  periods start from the day you receive approval. For example, if you
  begin flight training 30 days after you have been approved, you now
  have 335 days to finish. If you do not finish, a new training request
  must be submitted.

That's from the AOPA AFSP guide; the TSA's own AFSP webpage is a bit more difficult to use.


text:
Does a flight dispatcher need to update the flight plan during GTB and ATB?
Does a flight dispatcher need to recalculate and resubmit the flight plan to the pilot during ground turn back (GTB) and air turn back (ATB) scenarios?

Hello Raj, welcome to Aviation.SE! Please expand the acronyms to full words when you use the for the first time in your text. Also, we prefer the title to be in question form.

Does GTB and ATB mean gate turn back and air turn back?

Yes. Ground Turn Back and Air Turn back ..

Are you talking about the operative flight plan (used in the cockpit) or the ATC flightplan?

Operative flight plan to the pilots in the cockpit.

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sort: votes


text:
How can I convert a non-ICAO (Taiwanese) pilot's license to FAA or EASA?
So I'd like to know from somebody what's the process of converting a non-ICAO license. In particular, I'm trying to understand how a Taiwanese PPL can be converted to FAA PPL (I personally know people that have done it).
Taiwan is not a member of ICAO due to it not being recognised as a country due to China. So does that mean that a Taiwanese license is considered as a Chinese one and maybe that's why the FAA does the conversion?
I'd also be interested to hear about anyone who has tried to convert to EASA. I've never heard of anyone doing that.

Welcome to aviation.SE. It is a bit unclear what you desire. do you want to converto to FAA? to EASA? or you do not want to convert, but you want to know how to do it and why?

I want to know if it could be converted to any other license. I guess i have seen people convert to FAA but not EASA. This confuses me because Taiwan is not ICAO due to it not even being considered a country by ICAO and UN.

I can't tell you anything about the EASA regulations, but there seems to be no way to convert a non-ICAO license directly to an FAA one. 14 CFR 61.75 says (my emphasis):

(a) General. A person who holds a foreign pilot license at the private pilot level or higher that was issued by a contracting
  State to the Convention on International Civil Aviation may apply
  for and be issued a U.S. private pilot certificate with the
  appropriate ratings if the foreign pilot license meets the
  requirements of this section.

As you said, Taiwan is not a signatory to that convention, so the FAA can't issue a license under 61.75. (61.153 allows converting a foreign ATP license to an FAA one, but it also requires that the license come from a contracting state.) I couldn't find any special regulations or exemptions for Taiwan on the FAA's site.
You said that you know people who have "converted" a Taiwanese PPL to an FAA PPL, but do you know exactly what they did? My guess is that they applied for a regular FAA license, and used their flying time in Taiwan to count towards it.

Thanks for the answer. He did the regular procedure of going to an FSDO and converting. So it seems that its possible to convert his PPL. The thing is taiwan was one of the founding members but was kicked out in 70s by china who claim that taiwan is the same as china. So the way i can understand it is that they would consider taiwan CAA as part of china CAA (this is the only reasonable explanation) . Being that China proclaim it to be one country, so maybe FAA accept it as a China license. Does anybody have any FAA contact that could confirm?

@mizzu Hmm, maybe there is in fact some exception for Taiwan, even if it isn't documented anywhere. That would let the US convert licenses 'quietly' without annoying China, but that's pure speculation.

Actually I think I just found the answer in this document: http://users.ox.ac.uk/~sann2029/Chinese%20JIL%208%20(2009),%20135-159.pdf

It says that taiwan therefore is considered to be part of china as well as the airspace in the eyes of the ICAO. Therefore if understood correctly it should be the same as getting a Hong Kong License (HKCAD) which although has a separate caa from china is still considered as part of china icao membership.

@mizzu, write your finding as an answer (not a comment as you did), please. That way you can get reputation for it and can eventually accept it if you are confident of the results of your own research.

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text:
What is the purpose of the Boeing SCAP module?
The Boeing SCAP module will be used to calculate the takeoff and landing performance data. But what exactly does the Boeing SCAP module contain and how is it used to calculate takeoff and landing performance for the aircraft?

Sorry Fed, I Dont have any reference for this ... I am just looking for the parameter will be required to calculate the take off and landing performance for the aircraft

The Standardized Computerized Airplane Performance (SCAP) interface is a standardised model for calculating aircraft performance of air transport category aircraft. The SCAP standard is developed and maintained by IATA and leading aircraft manufacturers.
From the IATA website:

One particularly complex requirement is the calculation of takeoff performance data. Most airlines use programs supplied by the airplane manufacturers for each airplane type, and interface these programs with their own computer operating environment, airfield database, and data output format requirements.
In order to reduce the development effort for the airlines
  (interfacing with different airplane manufacturers' programs) and
  manufacturers (meeting individual airline requirements for
  customer-specific features) there are clear advantages in
  standardizing as much as possible the input/output features of these
  programs, and also data formats.
Having recognised this potential benefit, IATA, ATA and several
  manufacturers set up a Standardized Computerized Airplane Performance
  Task Force (SCAP TF).

A  SCAP module is a FORTRAN program that implements the SCAP standard. It takes a set of well defined inputs to produce a well defined output.
The advantage of this approach is that it is possible for a third party to design a software program that requires aircraft performance data without having access to this proprietary data. The software can then be used for aircraft performance calculations of any aircraft as long as it has access its SCAP modules.
There are 6 SCAP specifications for Takeoff, Landing, Climbout, Inflight, Noise and APM (Aircraft Performance Monitoring).
I don't have access to the official SCAP specification, but from what I can find online I gather that for take-off performance the following is required:

TODA (Take-Off Distance Available)
TORA (Take-Off Run Available)
ASDA (Accelerate Stop Distance Available)
runway elevation
runway surface condition
runway heading
wind speed and direction
ambient Temperature
QNH
MEL/CDL items affecting performance
max take-off thrust and reduced (FLX) option
anti-icing options
flap settings
V1/VR (fixed or opt)
V2/VS (fixed or opt)
minimum acceleration altitude
actual take-off weight

I gather the landing SCAP module needs:

LDA (Landing Distance Available)
runway surface condition
runway elevation
runway heading
wind speed and direction
ambient temperature
MEL/CDL items affecting performance
approach type (CAT I/II/III)
anti-icing options
presence of residual ice
flap setting (fixed or opt)
expected landing weight


text:
What is the upper altitude limit for propeller driven aircraft?
Is it possible to fly 100 kilometers with a propeller driven aircraft? and if not what is the limit of a propeller driven aircraft?

100 Mm (megameters) is a third of the way to the moon, 100 km is already considered space

The altitude 100 km is also known as the Kármán line. It is also called the 'edge of space'. No aircraft can fly higher than this line. Because of the sparseness of the atmosphere they would have to go so fast in order to generate lift that they would reach orbital velocity. Therefore they would be spacecrafts, and not aircrafts.
As you can see it's a theoretical limit, in practice aircraft fly a lot lower than this.
So in effect, no it's not possible for a propeller driven aircraft to fly above 100 kilometers altitude, but this is also true for any other kind of propulsion.
About your other question, I believe it's already answered quite nicely here: How high can propeller airplanes fly?

this is also true for **any** other kind of propulsion that's quite wrong.

The Kármán line is not necessarily at 100 km (330k ft). It depends on the aircraft and would mostly be lower.

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text:
How would an airplane land if the on-board radio breaks down?
Is there a special protocol for a situation where the on-board radio communication equipment suddenly breaks down? Would the pilot be able to land on a busy towered airfield or would they be forced to look for a small landing strip in the middle of nowhere?

It is important to remember that it is often difficult to diagnose radio malfunction during flight. Therefore it is hard to know whether you have a malfunctioning transmitter, a malfunctioning receiver, or perhaps both. You also have the point-of-failure that is the headset you are using. For this reason it is quite usual to carry a spare headset and the first piece of diagnostics is to swap headsets. Many General aviation aircraft also have a handheld microphone and speakers integrated into the radio stack.
Once you've ascertained that its the actual transmitter and/or receiver that has failed it is common to set the transponder to 7600. For landing, the relevant detail for the procedure to follow is contained within Chapter 4, Section 2-13 of the AIM:
Receiver Inoperative

Remain outside class D airspace until the direction of air traffic can be ascertained
Notify the tower your type, position, altitude, intention to land and request you are controlled with light signals
At 3 -5 miles, advise the tower of your position and join the traffic pattern
Watch the tower for light signals
Continue to advise of position (downwind, turning base, etc)

Transmitter inoperative

Remain outside class D airspace until the direction of air traffic can be ascertained
Monitor the primary local control frequency as depicted on sectional charts for landing or traffic info.
Look out for light signals that may be directed to your aircraft
During hours of daylight, acknowledge tower transmissions or light signals by rocking your wings
At night, acknowledge by blinking landing of navigation lights

Transmitter & Receiver inoperative

Remain outside class D airspace until the direction of air traffic can be ascertained
Maintain visual contact with the tower and acknowledge light signals as above.

The light signals used for an aircraft in flight are as followssource:
Steady green - Cleared to land
Flashing green - Cleared to approach airport, or return to land
Steady red - Continue circling, give way to other aircraft
Flashing red - Airport unsafe, do not land
Alternating red and green - Exercise extreme caution
They can be seen in this video.

Now, how is the light from the light gun visible on a bright day? Aren't the window panes of ATC towers tinted? Do light guns use lasers? (but that would cause a lot more problems than it would solve I guess) :d

@shortstheory - I suspect they use high intensity bulbs. Historically, sodium bulbs probably - more recently I suspect they may have gone to LED. Short answer - I don't know!

Oh well, I guess I'll just post it as a question then...but thanks anyway!

@shortstheory I have seen a light gun in action, they're quite visible even in bright daylight.

@shortstheory A lot of the time, the controllers just walk outside the cab onto the catwalk that (usually) runs around the outside of the control tower. :)

How would this work in low-vis situations like fog? I imagine that radio failure in IMC is the worst-case scenario and the tower would have little choice but to act as if the plane had requested emergency landing.

@KeithS - If im not mistaken, one requirement to fly IMC is a backup radio - as in a whole other radio in case of malfunction.

@Jamiec - ...Which is why a 172's radios aren't all in one unit; they each have the Tx/Rx circuits for their COM and NAV halves. That makes so much sense now. I assume those radios have to be totally redundant right out to the antenna?

@KeithS The COMs have to be totally redundant anyway so you can use both at the same time, e.g. listening to weather updates on one while talking to ATC on the other.  The NAVs have to be redundant as well since many non-RNAV instrument procedures require two VORs (or VOR+ILS) to identify fixes.  The latter can share an antenna since they're receive-only, but the former couldn't transmit and receive on the same antenna at the same time, so they each must have their own.

@Jamiec: And what do you do if you do have a dual radio failure in full IMC?

@Sean Fly the aeroplane. Land as soon as possible. Or to put it another way, aircraft dont fall out the sky because their radio stopped working

@Jamiec I've seen the "exercise extreme caution" for alternating red and green a million times. but what exactly does that mean?? Do I hold in position, land, go around, etc, at my own discretion?

First thing you do when the radio breaks down is to start squawking 7600 (the code for radio malfunction) on your transponder.
Then when you approach the airport they will attempt to contact you first through radio (to see if you can still hear them) and you use alternate means of replying (sending IDENT on transponder, rocking your wings,...).
If you can't hear them they will use a light gun to convey messages.

Nice answer. Please add that pilot should also continue to transmit on radio, even when it has failed. It could be that the radio would transmits some messages.

... or that its their receiver/headset not working, and that they're transmitting just fine.

How does is the light gun used to transmit messages? Does it just send pulses of light (a la morse code)? I'm withholding a +1 till I get an explanation XD

@shortstheory - you might like to check my answer ;)

Okay +1s for both of you then :)

note that it is often legal to fly VFR even without a radio, so you don't have to squawk 7600

@falstro It's perfectly legal to fly VFR without a radio at uncontrolled fields, but it's not legal to even enter Class C or Class B airspace without first making radio contact with the controller, is it? Of course, if you have a radio failure and the only airport around where you can reasonably land is a controlled field, then you don't really have a choice, but you should squawk 7600 in that case.

@reirab - It's not legal to enter any tower-controlled space without a radio unless you had an in-flight radio failure. Even class D towers will want you to check in. Circling the airspace with your transponder on 7600 is the cue to the tower to haul out the spotlight.

Ratchet Freak has a good answer.  In addition, please pull out your cellphone and try calling the tower.  Somewhere in the bottom of my flight bag I have a small book with telephone numbers at towers.  If you don't have that, call Lock-Mart Flight Service at 1-800-wx-brief tell him you need the tower number.
Also remember that there is no requirement for a radio in an airplane.  NORDO (No Radio) aircraft are perfectly legal although in practice, they operate mostly out of non-towered fields.  Operating out of a towered airport is still possible, but it requires prior coordination with the tower.  Note please that flight in the SFRA around Washington DC does require a radio, and I have seen no procedures to allow for NORDO aircraft in this area.
If your radio stops working, one option is to fly to a non-towered field and do what the NORDO planes do: Overfly the field at a safe altitude, note wind direction from the wind sock or tetrahedron, note other traffic in the area, and fit yourself into the traffic pattern... and land.

Ever tried using a cellphone in a GA cockpit? I'll stick with the light signals!

@Jamiec it works quite well actually, depending on the aircraft of course. It might be illegal though.

@Jamiec You can get headsets with bluetooth connectivity for your phone now (heaven help us when someone taxis into the fuel farm because they were yakking away...) - Headset or not though, you should try to use all available means of communication at your disposal, and that includes a cell phone if you have one - maybe the person on the other end can understand you & at least relay your situation. Worst case scenario the phone will be unusable, but at least you tried.

Yeah, from what I gather, using a cell phone in the plane is still illegal (FCC if VFR, FAA & FCC if IFR).  Would I use a cell in an emergency, sure, if I thought it would help.  Is losing radios an emergency?  Typically no (although I can imagine circumstances where it might be)...

@BrianKnoblauch  Yes using a cellphone in a plane is illegal.  Does anyone reading this have knowledge of anyone being arrested or fined for doing so, in an emergency situation (or even one where safety is enhanced by using the cellphone as in this example)?  I believe it is de jure illegal and de facto legal.

@BrianKnoblauch "FAA & FCC if IFR" do you have a citation for this, in particular the FAA part?

I was a Junior Technician, later Corporal, Air Wireless Fitter in the R.A.F. in 1952-4, servicing Gloster Meteors' radios at a Flying Training School, and the signal then was for the pilot to waggle the aircraft's wings as he approached, or flew by, on his way to land, to show he could not contact the Control Tower. Then we had to fix it.
I was made a Corporal so that I could sign off our work on the aircraft out on the airfield, usually just tuning up to a new crystal to suit the frequencies of a distant Station. I got the promotion so the Sergeant could stay in the nice warm office: I didn't mind -- I was "/Paid" extra, nice for a "National Service Only" erk.
Happy days

The NORDO procedures are covered extensively in the AIM.  That is suggested reading for any pilot.
Cellphones are unlikely to work airborne, except close to the ground, and then in remote areas. Most cell systems will ignore a phone which brings up many towers, thereby preventing it from calling. Text might work better. Most pilots don't frequently call towers, but it is worth noting that many times the tower will not answer the phone, depending upon workload.  Calling FSS might work better, and they have more direct access to the tower through multiple means.
Many pilots carry a small transceiver, which is a good backup device. I use mine to get ATIS and Clearance prior to engine start.  At least that way I know it is frequently working.
A final point, if you do loose transmit capability, listen on the last frequency you were talking on. They will probably try to contact you, and you may likely get walked through communications changes until you are on the ground, listening only. That has happened to me. The radios lost transmit capability due to a failure in the audio panel.  Center handed me to approach, who vectored me to the ILS, and gave me a frequency change to the tower. Tower gave me taxi instructions. There was no need to transmit. No need to use a cellphone or anything else, and no need to dig out a backup radio.
But the starting point for understanding the procedures is to visit the AIM.
https://www.faa.gov/air_traffic/publications/media/aim.pdf

If your radio was broken during a flight, you could use a light or come up with a sign of help that a helicopter or other airplanes can see so that they can contact the radio people and rescue them to safety.

Welcome to Aviation.SE. There is already a defined signal if your radio equipment becomes unserviceable: Set your squawk to 7600 and wait for light signals from the Tower. In case you are IFR, there is defined procedures to be flown under lost comms situations.


text:
What are these ribbons on the wing of an MD-83?
What are these triangles and ribbons on this MD83's wing, and what do they do?

These are Ice Detection Tufts that check for ice. If they are stuck, it indicates there's a layer of ice on the wing, disturbing airflow and presenting a danger for the engines when it detaches. They are focused around the rear root side since this is most likely to be the spot with the most buildup, as remaining cold fuel below flows to this corner.
SAS had an incident with ice a few years ago in the Gottröra Accident (SAS Flight 751) that thankfully ended relatively well.

Are they purely visual in nature? Or are they connected to some kind of sensor?

Purely visual; I have never heard of digital wing ice sensors in use on commercial aircraft.

Why wouldn't ice build up on the leading edge first?  Certainly the fuel can't be any colder than the OAT?

@MikeFoxtrot I assume flight attendants are instructed to monitor them? I doubt they advise the passengers, "Hey, could yall make sure that stuff is still moving?"

@HCBPshenanigans Ground staff and/or pilots check that they are free before flight either by method of deicing fluid, weather or trying to move them.

@SHAF Leading edge is heated. To the best of my knowledge The fuel can be colder than the outside temperature, for instance after a flight or after a long night parked outside. Ice can form in certain temperature/ precipitation cycles.

@MikeFoxtrot Ah, so its mainly a ground check function, that makes sense to me now.  Thanks.

@MikeFoxtrot There are indeed digital wing ice sensors that are used on commercial aircraft, and there are a couple of different types.  Here is one of them.

Some more information about this particular item.
AD 2002-21-06 addresses the issues with detecting wing ice on the DC-9 series of aircraft (which includes the MD-83).
Some aircraft of this type have a wing surface ice detection system installed.

There is also a modification available that automatically moves fuel between the wing and center fuel tanks to prevent the supercooled fuel from accumulating in wing tanks.


text:
How does the ADS-B compact position report NL() function work?
I'm trying to decode ADS-B messages for a school project, and I cannot reproduce the results of the NL() function as described in this document. I've seen the same formula in several other documents. Also in this document is the reverse of this function - the NL is input, and the latitude "boundary" is the result. I cannot solve that either.
I've tried radians and degrees for the trig functions and several different calculation tools, and my result is never close to correct.
The "known" good inputs and result are from this document. I've also seen these inputs and results elsewhere, but no details about the equation itself.
The summary of the "known" good data set is:
$rLat = 10.2157745361328$
$NZ   = 60$
$NL   = 59$
The equation is:
$$
NL = int \left( 2 \pi \left[ arccos \left( 1 - { 1 - cos \left( {\pi \over 2NZ} \right) \over cos^2 \left( {2 \pi \over 180} \mid lat \mid \right) } \right) \right] ^{-1} \right)
$$
Can anyone help me understand this calculation?

I'm pretty sure latex is supported here, at least I've seen questions and answers with that format before...

@JayCarr; well almost, we have MathJax, it's supports a lot of LaTeX's equation-stuff, so just paste it, it might very well render just fine.

Hi Kevin welcome on Aviation.SE. I will try and help you, but from your description there is little to start from. What doesn't work, what language do you use? What is your background, what kind of school work is this for?

I added the equation - you might want to double check my work to make sure it's correct.

NZ should be 15,see this code https://code.google.com/p/adsb-pgr/source/browse/trunk/decsub02.c?spec=svn2&r=2

Compact Position Reporting (CPR) is a way of reducing the number of bits needed to transmit position whilst maintaining high position resolution (~5.1 meters for airborne encoding). Ordinary encoding would require 45 bits, but CPR uses 35 bits thus saving 10 bits. To do so, the world is divided up into a number of zones, both for latitude and for longitude and the position within the zone is encoded. Since transmitting the zone numbers would cost as many bits as that would have been saved a clever trick is used. Two types of encoding are used, each with a different number of zones. From the difference in zone coordinates between the two types of encoding one can determine which zone the aircraft is in. Therefore one needs to receive an even and an odd encoded message within 10 seconds from each other. Once the initial position decoding is successful every subsequent position message will give the updated position.
In CPR decoding, you first determine the latitude of the aircraft based on a pair of odd and even position messages received within 10 seconds from each other.
Latitude is always encoded in the same number of zones between the equator and the poles (15 for even encoding, 14.75 for odd encoding). Due to the roundness of the earth the numbers of zones used for longitude encoding depends on the latitude. Longitude even encoding uses 59 zones at the equator and only 1 at the poles. Odd encoding uses always 1 zone less, except near the pole where 1 zone is used like in even encoding. The number of longitude zones used for even encoding can be found by the $NL()$ function.
The transition latitudes, i.e. where the number of zones for longitude encoding change, can be calculated by the inverse $NL()$ function. This function can be used to create a lookup table of transition latitudes which is more efficient to use than the original $NL()$ function.
Without knowing anything about your implementation it is difficult to assist further, the formula is depicted correctly in your question. Note that input latitude is given in degrees, but the trigonometric function are assumed to work on radians hence the $\frac{\pi}{180}$ factor.
Edit: on further inspection I noticed a mistake in the formula: it should be :
$$
NL = int \left( 2 \pi \left[ arccos \left( 1 - { 1 - cos \left( {\pi \over 2NZ} \right) \over cos^2 \left( { \pi \over 180} \mid lat \mid \right) } \right) \right] ^{-1} \right)
$$
(note that the conversion from degrees to radians is corrected)
For those that really want to understand CPR coding and decoding in detail, you'll have to obtain a copy of RTCA document DO-260B or Eurocae document ED-102A (unfortunately they're expensive, I think about $700) and read Appendix A.1.7 and Appendix T.

where did you get that information from: Latitude is always encoded in the same number of zones between the equator and the poles (15 for even encoding, 14.5 for odd encoding). Especially the 14.5 for odd encoding instead of 15? Couldn't find it on the internet....

Hello Florian, welcome to aviation.stackexchange.com. I got the numbers from ED-102A or DO-260B. In fact the latitude coding is 360 degrees from the equator, over the north pole, then the backside of the earth and from the southpole up to the equator. Only the 0-90 degrees and 270-360 degrees are used. There are either 60 or 59 zones in the full circle, so 15 or 14.75 from equator to pole. I know realise that the 14.5 I wrote is a mistake, it should be 14.75. Thank you for notifying!


text:
Why did the tower at a Class D airport not set me up with flight following on the ground?
On a trip I had a few days ago I had contacted the ground controller at Santa Fe Municipal (KSAF), a Class D airport, to arrange for a VFR departure to the southwest and requested VFR flight following.  The ground controller said he couldn't arrange flight following and that I had to arrange that with Albuquerque Center while in the air.  I was taught (and encouraged) that you could get flight following while on the ground at a towered airport.  The tower was not busy at the time as I was the only plane talking to them and the airport has its own radar which I know the controllers can see in the tower.  In fact I had forgot to reset the transponder to 1200 and they called me out on that after I took off.  Is there any particular reason why they could not set up flight following for me on the ground?
EDIT:  The FAA has a program where they can contract out a control tower at an airport to a private company.  I just looked at this FAA website and Santa Fe is listed as a contract tower.  Could this be the reason why the tower could not arrange VFR flight following?

I was taught the same thing, yet at one particular towered D, they tell me the same thing: "Unable. Contact Approach for Services"

@abelenky:  Do you remember the name of that airport?  If so look and see if it is a contract tower.

Some towers have a data terminal that allows them to create and update flight plans*; these are typically the ones busy enough to have a separate Clearance Delivery frequency, but you'll find exceptions both ways.
Other towers do not and would have to call a facility that does, which takes that controller away from actually controlling traffic; these towers tend to be the ones with less traffic and lower staffing (maybe only one person on duty), so they're usually not going to do that for VFR requests.
However, all Approach and Center facilities have the ability to create and update flight plans* at every radar position, so it's usually no problem for them to set you up once you're airborne and talking to them directly.
(* VFR Flight Plans are not accessible by US ATC; when you get Flight Following, it is actually put in the computer as an IFR flight plan with a VFR altitude.)

VFR flight plans are visible...they just don't want to deal with them.

So if I understand you right, the towers without a data terminal would have to cancel an IFR flight plan after a plane has landed by talking with the approach controller and can't cancel the flight plan directly?

@DLH Yes. Ditto for giving an IFR clearance and release.

@PilotDan VFR flight plans could be accessible for search-and-rescue purposes if we call FSS on the telephone I suppose, or perhaps the center might have a terminal in a corner somewhere that can pull them up, but the flight data system that processes intra-facility flights in the NAS has absolutely nothing to do with a pilot-filed VFR flight plan.

You can request flight following from the tower but they are under no obligation to provide it and may not as is the case with flight following in general. At my local class D airport (KPNE) it’s hit or miss and seems to depend on who is on the tower.
I suspect that in some cases it’s simply more expeditious for them to deny the request and have you contact departure on the way out.

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text:
What is the format of the airport codes used in an ACARS OOOI message?
How are the departure and arrival airport codes in the OUT, OFF, ON and IN (OOOI) fields identified by ACARS and transmitted to ground systems?
For example, if my aircraft is diverted unexpectedly to Bangkok because of some emergency, will ACARS send the arrival airport code as BKK in the IN message? If so, how will it be identified?

I'm looking forward to an answer that includes definitions of OOOI. I mean, I get the Out, Off, On, In but what does it all mean???

@FreeMan "OOOI
(Out of the gate, Off the ground, On the ground,
and Into the gate messages): Pushback from gate,
wheels off, wheels on, arrival time at the gate, number
of passengers, wheel chairs required, etc."

@RedGrittyBrick - thank you sir! OOOI is certainly much shorter...

ACARS messages, unsurprisingly, use IATA airport codes.
See ARINC 618, 619, 620 etc.

- ACARS, Dr. Maarten Uijt de Haag, Ohio University

Thank you for the detailed explanation. So the ACARS automatically detects the airport code and appends with IN to the Gate Acars message?


text:
Do dispatchers ever update and re-send a flight plan to an aircraft in flight?
In what scenarios would the dispatcher recalculate the flight plan and send it to the pilot via ACARS after take-off?

As a historical note, before ACARs in 747-100/200 aircraft in the 1990s, we would be re-dispatched in flight using HF radio. Our paper flight plans had both our original dispatch destination that would be used to meet the reserve fuel requirements, and also a section with our re-dispatch destination to be used if our fuel remaining at the re-dispatch point was sufficient to meet the lower fuel reserve requirements from that point. This was almost always necessary for flights from JFK to Tel Aviv in 747-100 aircraft and often necessary for 747-200 aircraft.

Anytime the original routing or altitude is significantly changed, a new flight plan is calculated by dispatch and sent to the aircraft.  Significant changes could mean the aircraft may not have enough fuel to reach the destination. Even minor changes are routinely re-calculated and sent to the aircraft.
The pilots send dispatch the new routing and altitude along with an updated ETA and FOB(fuel onboard) for the next waypoint. The flight plan is then recalculated from that point and sent to the aircraft.
Routing changes can be due to traffic conflicts, airspace closures, military activity, volcanic activity, etc.
If for any reason the aircraft is unable to contact dispatch, the pilots must do their own calculations to insure they have enough fuel.
If the fuel remaining is marginal, the flight can be re-filed with a new alternate closer to the final destination. If conditions can be met, the flight might be re-filed with "no alternate".

By accident. Once the clearance is accepted and the flight is airborne it's up to the controllers and pilots to make any needed changes by coordinating.

There are other reasons. Redispatching flights is not an atypical practice as I understand it.


text:
What's the standard print size for a sectional chart?
I'm checking out the Dallas sectional in digital form for my own amusement. The image, in TIFF format, is massive. I wonder, if you buy one of these in printed form from the FAA (or your local GA office), what would be the sheet size of the map?

The New York Sectional (the only paper one I have on hand right now) is 20 3/4 X 59 1/4 inches. You are correct, they are large and printed on nice thick paper.

And take ages to fold for the first time!

But is helpful for beating back a lizard stowaway in a glider.


text:
What is the final approach fix altitude for this localizer approach?

As I'm looking at this approach chart, I see that the glideslope intercept altitude is at 1400 feet MSL. However, for the localizer approach, I believe we stay at 1400 feet MSL until RIZER, which we can then descend to our MDA. Is that correct?

The lightning bolt is FAF for precision approach. The Maltese cross is FAF for non-precision.

Coram is not the FAF for a non precision approach.  The FAF for localizer only is Rizer.  You must protect 1400 till Rizer then you may descend to the applicable MDA.

For localizer only, the step down is from 2000 to 1400‘ to the final approach fix.   The lightning bolt could be indicating that the glideslope for the precision approach is intercepted at 1400, but that’s not quite clear.   At the final approach fix, start your timer so you know when you hit the missed approach.  So for me, I would descend from 2000 ar Coram to 1400‘ and maintain until the final approach fix, then start the timer, and start the descent to the MDA which is 520 feet.


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Why doesn't the pressure inside an airliner push the doors open?
When flying at cruise altitude, the pressure inside the aircraft is higher than outside and that means a lot of air is pushing the door outwards. So how does the door stay in place and not open?
I'm not asking if a passenger could open a door, because that's almost impossible. I'm asking how does the door not open if there is a lot of air pushing it outwards?

I don't understand the question. Obviously doors can be opened, because people go in and out of them on the ground. If it's impossible for a passenger to open a door, what do you think is going to open the door? Most airliner doors (cargo doors generally excepted) open inwards (plug doors). Since there's a lot of force pushing the door outwards while the aircraft is pressurized, as you note, it requires a lot of force to pull the door inwards.

http://aviation.stackexchange.com/questions/15011/what-prevents-a-passenger-from-opening-the-emergency-door-on-his-own-will-mid-f

@ZachLipton My question is: in-flight, when the cabin pressure is higher than outside, how does the door stay in place and not open? Because there is a lot of air pushing the door outwards

What stops anything breaking when it's used? The fact that it's been designed to be used in the way that it's designed to be used.

Most of the airline doors are plug type- i.e. they are wedge shaped, so that the pressure differential pushes it against the fuselage and prevents opening. For example, the following image shows the Boeing 737 door.

Image from airteamimages.com
Note that the door is bigger on the inside, so that when it is locked in position, the pressure will push it against the fusealge, preventing it from opening. For opening the door, it has to be swung inwards before moving out.

Most of the airline doors are plug type - not for long.  Most of the current generation don't use plug doors.

@Simon Am I right in assuming that the door still latches into the frame under force from seal face? Can they be cycled to another position whilst the door is under the pressure differential at cruise?


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Can you be cleared to cross a runway *behind* an aircraft on takeoff?
Taking KEWR as an example, let us say that aircraft are taking off from 4L at N and landing on 4R.  Can I be cleared to cross 4L at say V, behind the aircraft taking off on 4L, without interrupting the departure flow? Or will the runway need to be clear full-length to let me cross it?

You can also be cleared to cross in front of an airplane in landing roll when it is clear that it can be stopped before reaching the intersection you cross. For example when the landed aircraft reports "speed down" to the ATC.

You can cross behind the aircraft on departure roll. This type of operation is done heavily at ATL, on the middle complex during an east flow. Aircraft will depart  runway 9L at M2, and arrivals from either of the south runway will cross behind them the whole time at M1, to get up to the gates.

At smaller airports with multiple runways, it is fairly common to get a runway crossing clearance behind an aircraft taking off (or landing). Sometimes the tower will ask you to cross "no delay" if there's another aircraft on short final so you don't get in the way of the arriving aircraft.

authors: Дмитрий Чакин


text:
What are upwash, downwash and propwash?
What exactly are upwash, downwash and propwash? My understanding is:

Upwash is the fluid (air) that initially flows over the wing
Downwash is the fluid going down the wing
Propwash is a byproduct of the thrust created by the propeller, and you would experience it ahead of the propeller

Is that correct?

See Why does the vortex created by wing affects its own angle of attack? for upwash and downwash. See SKYbrary for prop wash.

I don't understand the answer. Is upwash the flow of air over the wing that rotates upward and then when it turns inboard, it is downwash?

The wing turns air downward, this is the origin of lift, but air moved downward must be replaced by air coming from the underside of the wing. This exchange takes place in tip vortices. The overall upward displacement is the upwash. On the top/bottom sides of the wing, there are respectively an inward/outward flows which creates smaller "bound" vortices with corresponding upwash (ahead of  the wing) and downwash (behind). More here, see this picture.


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Where is the fuel vent on a Piper Archer TX?
I realized I haven't really noticed where the fuel vent is for the 2016 Piper Archer TX that I've flown, but I assume the fuel is vented from the fuel cap. Is that correct?

I dont know about the new ones but the older Archers had a small tube vent under the wing. It looks like a small straw sticking out near the tie down point.

They find interesting places to hide fuel vents. In my C-177, the vent is wedged between the aileron and the wingtip. Not a great design choice either as the vent used to get loose and jam the aileron, resulting in a clamp that has to be checked in preflight.

Refer to your POH or equivalent.

I've looked through it and can't find anything about the vents.

Try looking on the checklist. Does it say to check the vents when looking under the wings, or does it say to check the fuel caps (vents).

Is this a rental aircraft?

Nope, school's. I was just going to ask the CFI when I go, but I was just itching and curious to know now.

@RonBeyer The location makes a lot more sense if you remember that the fuel tank is a structural component of the wing on the PA28 series (it forms the leading edge between two ribs): The vent line is routed internally along the back of the fuel tank assembly & pokes out the bottom behind/outside the "wet" part of the tank, so when you remove the tank the whole vent apparatus comes out with it for "easy" servicing.

@voretaq7 It makes sense on the PA24/28, even the 172, but the 177 is a little odd. The tanks are in the normal position near the fuselage but the vent was routed all the way to the end of the wing between the tip and the aileron. I'm guessing that they did that to keep the vent out of the air stream, and there are no struts to hide it behind or "umbrellas" on the fuel caps like the PA28. The bigger issue became the tube coming loose and jamming the aileron, which resulted in an AD to install retaining clips not included from the factory, which are little more than plastic wire clips.

There are four main tank vents on all PA28 (Cherokee/Warrior/Archer/Dakota/Arrow) aircraft, two in each tank.
The most visible vent is under the wing at the outboard aft side of the fuel tank (near the jack points):

This vent is a small tube that protrudes from the bottom of the wing and is cut at an angle facing forward to provide ram air pressure into the tank. It is the vent you typically check during preflight to ensure mud wasps or similar haven't built a nest in the conveniently bug-sized tube.
The second vent is located in the fuel cap: It is an orange rubber "umbrella" in the center of the fuel cap which covers breather holes.

If pressure in the tank becomes negative (because the under-wing vent is obstructed) the umbrella will be forced open by air pressure and allow air into the tank through the fuel cap.
This vent is not normally "checked" during preflight except to ensure that the rubber umbrella is in fact there. A failure of the rubber "umbrella" is one common way that water finds its way into the fuel tank (the other being a failure of the O-ring around the perimeter of the cap).

I thought that piece on the bottom of the wing is called a scupper drain. Am I mistaken?

@nyorkr23 I've never heard it called that - the Piper parts manual identifies it as element 75 in the fuel system diagram ("VENT ASSEMBLY - Fuel tank" left & right as appropriate). It does function as a scupper drain if the tank is overfilled though: If you look carefully at the filler neck inside the tank you'll see a hole where the vent tube connects, and if the tank is filled enough for fuel to reach that hole the excess fuel will be dumped overboard through the vent line.


text:
What would be the airspace class when no vignette is visible on a US VFR sectional chart?
If a airport doesn't have a pink vignette nor blue vignette on a VFR sectional chart, but the airports around this particular airport has pink vignettes, would this airport's Class E start at 1200 AGL or would this airport be Class G up to 14,500 MSL?
For example, Grayson County (M20)  - circled here in red - has no pink vignette but other airports near it do:

Do you have a specific example. There are very few areas in the US that are Class G up to 14,500 ft. If you don't see blue vignette it's most likely 12,000. See this question

Garyson Airport, M20 on the sectional. And did you mean 1,200 feet?

Yeah, sorry. I did mean 1200.

Could you confirm M20 Class E starts at 1,200 feet?

Yeah, that's definitely 1200 ft. There's No 14,500 anywhere near there.

So basically, anything that is with a blue vignette is most likely Class E starting at 14,500 feet MSL?

If you look at the surrounding airports they are inside 700 ft vignette. Outside that vignette it is 1200, unless you see blue vignette. There is no place where it jumps from 700 ft to 14,500 ft

Haha, yes I understand. I'm just concerned if there are random airports that just have Class G that pops all the way up. Best way to figure it out is look at elevation and if there are any vignettes. Class E is a pain.

Yes it is. The vast majority of the country is within 1200 ft area. If you don't see any blue vignette then the entire map is technically within the blue area. It would have been easier if they had done it the other way around, where 1200 is considered norm and 14,500 put inside a vignette. But that's not haw the FAR's are worded

The legend in the chart will tell you where class E starts outside of the vignettes. See the answer here: http://aviation.stackexchange.com/questions/34793/why-is-it-assumed-on-sectionals-that-the-area-outside-of-a-700-agl-class-e-echo/34794#34794

The vast majority of US airspace where no more restrictive airspace exists has class E starting at 1200ft AGL with class G below. You can see an area of class G up to 14,500 MSL just north of the TCS VORTAC near to Truth or Consequences, NM which is south-southwest of KABQ. There are several other areas in the 48 states as well. As long as the airport you are talking about "near airports with pink vignettes" is not under the pink vignette, then G extends to 1200 AGL. If it's inside the pink vignette, G extends up to 700 AGL. Of course, if it lies within a pink dashed line indicating class E to the ground at such a nearby airport, then the airport you mentioned does not have any class G above it either.
One other helper I use - if you see any federal airways nearby and there is no blue vignette indicating the presence of class G up to 14,500 MSL then you can usually safely assume it is class E above 1200 AGL if no other more restrictive airspace overlies the area. Federal airways are 4 nm wide on each side or the course centerline and always exist within controlled airspace (class E or better). These are labeled on a VFR chart as the light blue lines with V-number labels - also called Victor airways. As such, V 49 west of the M20 airport you mentioned would be an example and is pronounced "Victor forty-nine".
At M20 - the airport you ask about - the airspace up to 1200 ft AGL is class G, then class E up to but not including 18,000 ft MSL, then class A at 18,000 MSL up to FL600, then class E to space.

You are correct. M20 is in Class G airspace up to 1200 ft AGL, then Class E airspace from 1200 ft AGL to 17,999 ft MSL, then Class A airspace from FL180 to FL600, then Class E airspace from FL600 and above.
As noted earlier more and more Class G airspace from surface to 14,500 ft MSL are being converted to Class E airspace due to increased traffic congestion in the CONUS.  Examples can still be found in the southwest.  Large swathes of Class G still exist in Alaska due to vast remote sections of territory there.

Another way to check the airspace around an airport is to look it up in the Chart Supplement.
M20
You will notice that there is no airspace line in the airport data. That usually means it is Class G. All of the other airspace types (E,D,C,B) are listed e.g.
KOWB
AIRSPACE: CLASS D svc 1200–0400Z‡ other times CLASS G.
KBWG
AIRSPACE: CLASS E
If you see clearance delivery, ground, and tower frequencies then, at least for most of the day, the airspace is not Class G.


text:
Does airspeed affect the critical angle of attack on an airfoil?
I understand that the critical angle of attack is mostly affected by the shape of the airfoil.  However does the airspeed of the airfoil or the density of the air affect the critical angle of attack?  I would think that the airspeed or air density would affect the laminar flow properties and could change the critical angle of attack of that airfoil, perhaps only slightly.

Could you perhaps wait with accepting the answer to attract more answers? I've been wondering about this exact question, but I'm hoping there will be some more quantitative answers than the current one.

Yes, through the Reynolds number
The Reynolds Number is a function of airspeed and is used to predict the transition from laminar to turbuluent flow:
$$
{Re} = \frac{\rho v L}{\mu} = \frac{v L}{\nu}
$$
The critical angle of attack is affected by the separation of the boundary layer, which, in turn, depends on the laminar or turbulent qualities of said layer.
Do note that the Reynolds number of the layer is not the same as the Reynolds of the wing, although they are related.
EDIT: to visualize the efect, see for example the following graph (thanks @MikeY)
Quite a bit of experimental work on low airspeed performance was done by Selig et al. at the UIUC, and their website maintains a significant catalogue of airfoil datapoints.
Another resource worth checking is Airfoiltools,  which provides graphed Xfoil polars for an extensive seleciton of airfoils. Do keep in mind Xfoil´s prediction of flow separation points is numerical and therefore may not converge with reality at all points.

Nice diagram...https://www.researchgate.net/profile/Ravi_Kishore16/publication/257011613/figure/fig1/AS:559922219843584@1510507268070/Influence-of-Reynolds-number-on-airfoil-NACA-0012-behavior-Musial-and-Cromack-1988.png

If this diagram represents something about velocity, why are neither the X or Y axes labeled with anything having to do with velocity?

@RyanMortensen velocity is subsumed into the Reynolds number, which appears as the coefficient generating the curve family.

Although an increase of velocity 2 orders in magnitude is a bit much, there are differences in doubling speed for a given Reynolds number. Good graph. +1

@RobertDiGiovanni No need to increase velocity by a factor of 100, as the chord also plays a role; this is another reason why wingtips often have different airfoils. The effect is most pronounced in small model planes, which fly close to the flow transition $Re$ anyway.

And horizontal stabilizers.

@RobertDiGiovanni meaning HStabs on airliner-size planes? No, not really.

From a pilot’s perspective, no. Stalling will occur at critical AoA at any airspeed or altitude.
Planes fly because of Lift being equal or greater than weight. Lift is the result of this formula:
$$ L = \frac{1}{2} \cdot \rho \cdot v^2 \cdot C_L \cdot S $$
(ρ=density, v=airspeed, CL=lift coefficient, S=surface area)
CL varies directly with AOA. The more AoA, the more CL... until the point where the boundary layer separates, and no Lift is generated. At that point the maximum CL, or CLmax is attained. The relationship between AoA and CL is obtained empirically for each aerodynamic surface.

The more amount of airspeed, the more Lift.
And the more AoA, more Cl, and therefore, more Lift.
But beyond the critical angle of attack, regardless of the airspeed, you stall.
You may think that you can go beyond the AoA and still be flying and climbing, but that is going to be a product of the thrust, that, with the acceleration is going to modify the direction of the relative wind. Remember that the AoA is measured against the relative wind, so when you apply thrust, you are modifying the "point of view" of your angle.
Regarding density, or altitude, you have to consider the difference between the True Airspeed (TAS) and the Indicated Airspeed (IAS). With a same IAS, at a higher altitude you are getting more TAS. This is because IAS is a function of the dynamic pressure (q), that depends on the density (ρ):
$$ q = \frac{1}{2} \cdot \rho \cdot v^2 $$
As you see from the equation, if you lower ρ, or increase altitude and still keep your IAS, you will be obtaining more v (airspeed generating the same amount of Lift).
But CL is not depending on density, and the CLmax is obtained at the same angle of attack at different altitudes.

Welcome to Aviation.SE! Could you explain further how you came to this conclusion? Just as with drag,, the lift coefficient is dependent on Reynolds number so the Cl/α curve is valid for a certain Reynolds number (and therefore speed).

"Stalling occurs at a specific AoA at any airspeed or altitude" is simply not correct, I´m afraid, see my answer.

@fooot: I interpreted the question from a pilot's perspective, where the pilot has no control over the Reynold's number, but on the AoA, surface configuration (ailerons, spoilers...) and speed. My point is valid from this perspective, where the pilot needs to know that he can stall the plane at any speed or altitude at a specific AoA. That's why planes have an AoA sensor. But of course, from a design perspective of the airfoil, you are correct.


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Why wing ribs have diagonal struts
Why do ribs have diagonal struts,why not vertical as seen in airfoils of Citabria airplane?

The diagonal struts are best to transfer shear. Imagine an upward pointing load at the nose. In order to get that load transferred to the fuselage, it first needs to be transferred to the wing spar. Imagine now that you hold the rib firmly in your right hand where the spar is and press the nose upward with your left hand. With parallel struts, the rib will have much less stiffness and the nose load will deform the rib easily. With the diagonal struts, the rib is much stiffer when that particular nose load is applied.

In #1 the struts are strong in compression, but the joints are weak in torsion, so triangles are used.  In #2, the joints are strong in torsion (in the rib plane).

I have a follow up question @PeterKampf. So if parallel struts are better for bending stress, and the diagonal struts better for shear stress, would some aircraft combine the two or would that create issues with weight? Would those new honeycomb structures they use on some planes be a compromise between the two?

@Jihyun: Many aircraft do this already. Ribs in large aircraft have round holes so the remaining structure will keep the upper and lower flange of the rib apart (good for bending) and transfer shear as well (good for transferring lift). Those holes will save some mass, but with larger aircraft there is no way around a heavier structure.

@Jihyun:  Regarding honeycombs: They are used for three functions: To transfer shear between the top and bottom layer, to keep them apart (good for bending) and to prevent them from buckling. What kind of load those panels can transfer is determined by the fiber direction in those layers.

I see. Thanks for the thorough answer!

This isn't really aviation-specific, it's just good structural engineering practice. Triangles in structures are strong because they don't bend: whichever way you apply load to the triangle, the load is along the members. With a rectangle, any load that's not perfectly aligned to the structure will bend the joints and squash the rectangle into a diamond shape, which will then squash flat easily. You don't want any rectangles in your structure anywhere.

@DavidTeahay What is that link supposed to relate to?

That the wing ribs of some planes(maybe few) is actually rectangular and not in triangles(truss)....is this a design flaw or can I just go with that kind of wing rib in the design of my own wings? @Dan Hulme

@David Teahay: You could of course design your wing using rectangles, but it would probably be heavier for the same structural strength.

@DavidTeahay Those google results don't show that at all. The pictures there show two forms of rib: ribs constructed of triangular shapes (such as your new picture 1) and solid ribs with holes in the middle to reduce weight (such as your new picture 2, though more of the examples have circular holes). The latter will generally be heavier for the same strength, but might be easier to manufacture. If you try to build a truss without triangles, you'll need so much reinforcement at the joints it'll end up heavier than a solid rib anyway.

http://www.russellw.com/logbook/citabria_wings/LH%20tip%20inward.jpg

@DavidTeahay Stop posting the same picture. As I already said, this is a solid rib with holes in, not a rectangular truss. It's still worse (heavier for the same strength) than a triangular truss.

It's a truss. Ideally, all of the beam elements in a truss are in either tension or compression, without major bending loads. This offers the lightest weight for given load.
This same design is commonly seen in the open in steel bridges, construction cranes, and spacecraft. Triangles that a truss consist of cannot be bent without changing the elements' lengths. If all of the struts were vertical, the structure would consist of rectangles, which can be easily deformed by bending the joints.

It's to make the wing stiff and strong in torsion. For torsion stiffness, you want two things. First and foremost, you want as much material on the outermost fibre as possible. The further the material from the neutral axis, the more it contributes to torsion stiffness due to the moment arm from the neutral axis.
Secondly, you want the cross-section to remain undeformed. Consider for example a matchbox 'shell', which is in theory very stiff for torsion. However, if the forces do not enter the matchbox exactly right, the open ends will deform into a diamond shape and will twist easily. This shearing into a diamond shape can be prevented by placing baffles in the matchbox.
This is in fact the very reason wings have ribs: so that the wings maintain their cross-section by transferring shear loads due to torsion from and to the top and bottom skins of the airfoil. In terms of stiffness, thin plates would be ideal, but these are prone to buckling so a strut structure is used instead. The best way to transfer shear loads with struts is to use diagonals.
So, in short: the diagonal struts transfer shear loads to increase torsion strength and stiffness of the wing.

You think ribs are mainly for the prevention of buckling? Then the vertical struts in the first picture would work just as well. I'm afraid you are not really answering the question at all.

@PeterKämpf Not at all. Stiffness and buckling are separate phenomena. I'm saying the diagonal struts are added for torsional stiffness to increase the eigenfrequencies, similar to adding baffles to a tube structure.

No, for torsional shear the ribs should be built similar to the wing skin - after all, they are part of the torsion box. With those puny struts they are certainly not up to the task. In other words, the rib shown is from somewhere internal while an end rib (which is part of the torsion box) would look much beefier.

I understand your second point but not the first. what do you mean by "outermost fibre" and what is "neutral axis"?....are you talking about the neutral point of an airplane? @Sanchises Pls elaborate

@DavidTeahay The 'neutral axis' is the line along which the cross-section of a structural member rotates if a pure torsion is applied (for a round tube, this would be exactly in the middle). With 'outermost fibre' I mean to say that you want as much material as possible as far away from this neutral axis. Material further away from the neutral axis contributes more to torsion stiffness than material near the neutral axis. Think of the difference of twisting a string or a tube of equal mass: you will find that the string is easier to twist because the material is closer to the neutral axis.

@Peter Revisiting this answer, I'm inclined to partially agree; that is: I based this answer on another design that used baffles to increase torsional rigidity, however, this concerned a bent tube (for which a single end plate would be insufficient). In this case, baffles such as these can serve the same function of a torsion box end plate (albeit slightly less efficient if the wing is sufficiently straight, and of course this presumes the wing skin itself serves as a torsion box / structural member). I suppose then this answer only applies for outlandish designs such as box wings.

A top picture of number 2 might have been a bit better, but notice the "puny" cross struts run both ways, much like in the wing of an old time biplane.
Their function would be help prevent the wing from
collapsing BACKWARDS as a result of excess airspeed.
(spokes in wheel rims may also seem puny, but the rim
cannot come out of round because the spokes, 90 degrees from the load, cannot be pulled apart).
This seems to be a bit of sensibly added insurance,
but no, they probably would not add to the torsional
stiffness of the wing.
Regarding the design of the ribs themselves, as mentioned, 1 is a classic truss.  2 has the center of
the rectangular rib filled in with metal and the center circular portion and edges made as an I beam for strength.  1 of wood, 2 of stamped metal.
As far as making a stiffer wing torsionally, might consider replacing aluminum skin with plywood or some type of composite material.  Others may know how better to do it within the wing.


text:
Do elevators have cambered airfoils inverted or symmetrical airfoils
Do horizontal stabilizer of ga planes e.g cessna 172(or ultralights e.g lazair) have cambered airfoils inverted to provide the tail downward force or do they have symmetrical airfoils at negative aoi to provide the downward tailforce during flight.

I would think this answer will vary depending on if you are asking about stabilators or elevators both of which appear on small GA planes. You may want to specify which one you are asking about.

I think the design may vary for each manufacturer, and even for each model. You may restrict your question to specific aircrafts

@ManuH How is the OP going to know this without knowing the answer to this question? I'm sure generalisations can be made and if not, that's an answer too!

Generally they are either symmetrical airfoils (like NACA 0012) or flat plates (especially on fabric airplanes).  The horizontal tail doesn't have to work as hard, or operate over as wide an AOA range, as the main wing in generating downforce, so you can get away with what amounts to, on most fabric airplanes, a slightly streamlined sheet of plywood.
There are some specialty exceptions. The Zenith 701 family of homebuilt STOL aircraft are unusual in having a heavily cambered flat bottom airfoil for the tail (actually, flat topped, since it lifts downward and is installed "upside down").  They did this to get as much down force as possible at very low speed and since the airplane is a bit of a slug anyway, the draggy tail airfoil wasn't so much of a penalty.
A homebuilt I had started on but never got anywhere with, called the Pegazair, had something similar, having ripped off a few of the 701's design elements.
Also, it was a practice at DeHavilland Canada to use tail airfoils with a bit of negative camber to improve down force at low speed on their STOL designs.  I'm sure there are others.


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What is the lift-to-weight ratio during takeoff for an ultralight?
Lift to weight ratio at cruise is 1:1, what is the average lift to weight ratio during takeoff (rotation) of an ultralight with 2m/s as rate of climb?

Your (lift to weight ratio) load factor is less then 1 during climb.
In a climb or descend only some of the weight is counterbalanced by lift, another part is counterbalanced by thrust. So lift is less then weight in a steady climb. (But Thrust is greater then drag) To calculate the exact Lift to weight ratio we will need more information.


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Pros and cons of a pull-push propeller configuration
What are the pros and cons of a pull push propeller in comparison to a counter rotating propeller?

By counter rotating do you mean both in pull configuration?

And by counterrotating, do you mean one propeller immediately behind the other as in some later P51 configurations?

On an ultralight? I would think biggest CON is the weight of a structure to hold all that mass, vs small engines just on the wing.  Someone a while back, and I can't find a picture of it, had a small plane with small twin engines, that had a very open canopy, intended for nature shoots.  Drawing a blank on the name of it. Was an interesting design.

Take a look at some of the configurations here [1]  [1]:https://www.google.com/search?q=ultralight+twin&tbm=isch&source=iu&ictx=1&fir=ZEBebWpSkIfB-M%253A%252CD5pyd5RCZYONLM%252C_&usg=__gp48hX2PiFstvq7ZxNFUl9iDHqc%3D&sa=X&ved=0ahUKEwjH-oqStY3bAhVDq1MKHY9BDMAQ9QEIpQEwBg#imgrc=ZEBebWpSkIfB-M:  I think I might be thinking of the Red & Yellow pusher in the 4th row  [Air-Cam][2], that's the one  https://www.pilotmix.com/images/ac_images/crop/optimized/385_886eb87452a5cacaed28104680a83b63.jpg  [2]: http://www.aircam.com

center of gravity is a serious concern, too.

Is nobody going to comment on that registration...?

don't know about the general concept, but looking at the photo, I wouldn't want to fly something with center of thrust and center of mass so high.

Pusher-puller multi engine vs single or multi engine contra rotating propellers:
Pros:

Ideal configuration for a multi engine aircraft as it eliminates thrust asymmetry in the event of an engine failure.
As a consequence, improves safety during takeoff, landing and low speed, low altitude flight.
Can be a useful arrangement for multi engine flying boats.
Does not require oppositely geared engines nor heavy gearboxes.

Cons:

Engine crankshaft on the A/C centerline requires a more unorthodox airframe to accommodate which can present additional design challenges.
Does not fully eliminate propeller slipstream yaw depending on the aircraft layout.
Does not eliminate or reduce yaw from P-Factor.
Can be noisier inside the cabin than a twin with wing mounted engines.
Aft mounted Engine can be more difficult to cool due to indirect ram airflow.
Disrupted airflow from fwd propeller reduces the efficiency of the aft propeller.

You might want to add the lower efficiency of the rear propeller in a tandem configuration (when both are running), because it works in flow accelerated already by the forward propeller. This is especially noticeable at low speed.

A push-pull arrangement with a "puller" propellor in front of a "pusher" can give rise to problems of aerodynamic interference with the rear propellor at higher power levels, this was a problem with the Dornier 335 fighter.

"can give rise to problems of aerodynamic interference with the rear propeller at higher power levels"......the engine to be used is 6.5hp...will there still be aerodynamic interference?

Assuming that you're talking about a microlight aircraft or something similar, I would not expect serious problems. The Dornier 335 had two Daimler Benz DB603 engines with 1800 to 2000 hp each.

"aerodynamic interference with the rear propellor" - could you please be more precise? Any sources?

Offhand, I can't be too precise but if you think about it, a tractor propellor creates spiral pressure waves behind it and this will affect the functioning of the pusher propellor when it's blades intercept these pressure waves. If nothing else, the efficiency of the pusher propellor may be reduced. In the Dornier 335, the situation was complicated by the fact that the pusher propellor was mounted slightly higher than the front tractor propellor.

The book Hitler's Luftwaffe by Tony Wood and Bill Gunston mentions the interference problem without going into more detail. I expect this would have taken the form of vibration and possibly fatigue induced failure if this was severe enough.


text:
Where can I get a copy of my notice of disapproval from a failed checkride?
I can't find the notice of disapproval that I was given after I failed the oral portion of my private pilot checkride. I have looked EVERYWHERE.  What do I need to do?  Is there something on IACRA or can I go back to the examiner?
I am fairly close to taking the checkride again (I switched instructors), and I am under the impression that I need to take that paper with me.
Any suggestions?

Hmm, that could be an issue: 61.43 says "Presents to the examiner for the retest the original notice of disapproval form". I would call the FSDO and ask them, but there's a good chance you'll have to redo it.

Redo what exactly?

I assume the whole thing: without any record of which areas you passed the examiner can't give you any credit for what you did pass. But as @voretaq7 said in his answer, if you never got as far the flight test then you may not have accumulated any credit anyway.

As far as I'm aware you can't pull the "completed" application from IACRA (with the examiner's notice of disapproval & failed areas on it). Much like with your knowledge test report you need to hang on to that original bit of paper.
I don't believe you NEED to bring the Notice Of Disapproval with you to a retest though.
61.43 says, in part:

f) If a practical test is discontinued, the applicant is entitled credit for those areas of operation that were passed, but only if the applicant:
  (1) Passes the remainder of the practical test within the 60-day period after the date the practical test was discontinued;
  (2) Presents to the examiner for the retest the original notice of disapproval form or the letter of discontinuance form, as appropriate;
  (3) Satisfactorily accomplishes any additional training needed and obtains the appropriate instructor endorsements, if additional training is required; and
  (4) Presents to the examiner for the retest a properly completed and signed application.

So you only need it if you want to skip over the parts of the test you passed. If you don't have it, or your retest is more than 60 days after your original failed checkride, the examiner will have to give you a full exam, with no credit for the stuff you already completed.

Given that you failed the oral there's probably not much "credit for those areas of operation that were passed" you can claim, so it probably doesn't matter much that you lost the original notice of disapproval. You're almost certainly getting a "full checkride" anyway.

This is a correct interpretation. The examiner isn't even obligated to give credit for the previously completed elements, although they almost always do.


text:
Is a complex airplane endorsement required to act as PIC of a plane without all three elements of the "complex airplane" definition?
Is an endorsement required to fly act as pilot-in-command of an airplane equipped with a constant-speed propeller, adjustable flaps, and fixed-gear?
This does not meet the complex airplane definition because it is not equipped with retractable landing gear.

No, you do not need a complex airplane endorsement for this airplane because it doesn't have retractable gear, unless it is a seaplane.
All three elements are required for that endorsement to be necessary for landplanes.
A jet-engine is not a controllable-pitch propeller, therefore it would not require a complex endorsement even if it has retractable gear and flaps.
However, the above would require a type-rating because it is a jet.
An airplane with a controllable-pitch propeller and retractable landing gear that isn't equipped with flaps would also not fit the definition of complex, so you are absolutely right.
See §61.31(e) Additional training required for operating complex airplanes
The definition for complex is in the Airplane Flying Handbook Ch 11,

“A complex airplane is defined as an airplane equipped with a
  retractable landing gear, wing flaps, and a controllable-pitch
  propeller. For a seaplane to be considered complex, it is required to
  have wing flaps and a controllable-pitch propeller.”

The definition for complex as shown in 14 CFR §61.1(b)(iii):

"Complex airplane means an airplane that has a retractable landing
  gear, flaps, and a controllable pitch propeller, including airplanes
  equipped with an engine control system consisting of a digital
  computer and associated accessories for controlling the engine and
  propeller, such as a full authority digital engine control; or, in the
  case of a seaplane, flaps and a controllable pitch propeller,
  including seaplanes equipped with an engine control system consisting
  of a digital computer and associated accessories for controlling the
  engine and propeller, such as a full authority digital engine
  control."

@Fishdoc - glad this helped you! Give it about 24 hours to see if other answers come along (you may find one that better answers your question, or that answers an additional question you didn't even know you had), then be sure to stop by by and give it a check mark as the accepted answer. That's the best way to say "thank you".

Also the distinction between controllable-pitch propeller and constant-speed propeller is significant. There are now aircraft that have constant-speed propellers, but have RPM automatically selected depending on power and air speed and don't have separate propeller control lever.

Ok, so it turns out there actually isn't distinction between controllable-pitch and constant-speed propeller. If the pitch is not fixed, it counts as complex aircraft even though with automatic control it might be easier to fly than with fixed pitch. It does add some failure modes though.

My understanding, BTW, is that jets in the sub-12.5klb weight class do not require a type rating to fly

@UnrecognizedFallingObject there's a good question idea. Go for it!

@RyanMortensen -- I stand corrected, actually -- it's a separate clause in 61.31

@UnrecognizedFallingObject yes §61.31(a)(2): "Turbojet-powered airplanes"


text:
Why is there such a long displaced threshold on 22R at JFK/KJFK?
This video seems to raise a whole bunch of questions, but here's one I have:
Why such a long displaced landing threshold on 22R at JFK/KJFK? (Scroll down to the 4L/22R section for details.)
Certainly Rockaway Blvd. is sufficiently distant from the end of the pavement (TODA) to allow closer landings.  And further upstream from that is Idlewild Park.  No residential areas for a good distance away from that runway.
That's a 3425-ft. displaced threshold on a 12079-ft. runway.  Is that some sort of record? ;-)

Have you considered the intersection with 31R?

@vasin1987 How would that intersection necessitate having such a long displaced threshold? (For the record, I am not a pilot.)

I am not a pilot too. My guess would be the displaced threshold used to be part of intersecting runway but at a point in time they feel having intersecting runway interupt their operation so they turn part of runway into displaced threshold. So both runway can operate without having to make both a LAHSO operation. This is my guess.

Its to maintain proper clearance over the park on approach from the north and avoid trees. They tried to extend the runway in 2013 but were ultimately denied due to minimum altitudes. Its covered in the October 2013 document Runway 4L/22R Improvements John F. Kennedy International Airport
They submitted for

Relocate the displaced landing (arrival) threshold on Runway 22R 3,316
  feet to the north.

but ultimately...

Two of the elements, the relocation of the Runway 22R arrival
  threshold 3,316 feet to the north and the relocation of the Runway 4L
  end (departure starting point) 460 feet to the north, would have
  resulted in aircraft being at lower altitudes than existing conditions
  over areas to the north of the runway, including Idlewild Park. The
  lower altitude of aircraft would have resulted in up to 800 trees in
  Idlewild Park becoming obstructions, as defined in FAA Order 8260.3B,
  United States Standard for Terminal Instrument Procedures (TERPS). FAA
  Order 8260.3B specifies the minimum measure of obstacle clearance that
  is considered by the FAA (the Federal authority) to supply a
  satisfactory level of vertical protection for aircraft operating at an
  airport. If the project described in the May 2012 Draft EA were to be
  implemented, up to 800 trees in Idlewild Park would have required
  removal to comply with FAA standards. The project described in the May
  2012 Draft EA has been modified to avoid this impact as explained in
  the following paragraphs.
In order to minimize the number of trees to be removed from Idlewild
  Park, the Port Authority made the decision to redefine the proposed
  project as described in Section 1.3 of this Revised Draft EA. The Port
  Authority no longer proposes the relocation of the Runway 22R arrival
  threshold 3,316 feet to the north. The arrival threshold on Runway 22R
  would remain in its existing location, which does not result in lower
  landing elevations north of the Airport.

Ye link breaketh.

A runway threshold closer to Rockaway Blvd. would also mean that planes would be flying much lower over the residential communities to the north. The planes are already very low over those communities. Idlewild park is not a sufficient buffer. The residents protested moving the threshold closer to Rockaway. They also protested extending the runway end closer to Rockaway.


text:
Why are the outboard Krueger Flaps retracted before the others on the B747?
In this video, I noticed that the outboard Krueger Flaps on this Boeing 747 were retracted while the middle and inner sets remained extended (refer to 3:22 - 3:30 in the video).  My questions are:

Why the staggered retraction?
Does each leading-edge flap configuration have its own unique setting on the pilot's flap-control lever?
On approach, are the leading-edge flaps extended in a reverse staggered order?

Please note that I have already reviewed the below Aviation SE questions (and their answers):

What are the benefits of using Krueger Flaps?
Is it possible to takeoff without flaps extended?
Is the A320 family allowed to take off with the flaps retracted?
If flaps are used on takeoff, when should they be retracted?

But I haven't found anything related to how flaps (leading- or trailing-edge) might be extended or retracted in a staggered fashion like this.

This is from memory and applies to 747-100/200 aircraft, which I last flew in 1999. The video shows, I think, a -400. However, to the best of my knowledge, it has the same flap settings.
The flap selection handle detents and resulting device positions are:

0° — leading and trailing edge retracted
1° — leading edge inboard of the engines extended, trailing edge up
5° — all leading edge extended, trailing edge 5°
10° — all leading edge extended, trailing edge 10°
20° — all leading edge extended, trailing edge 20°
30° — all leading edge extended, trailing edge 30°

At least one carrier, TWA, but maybe others, ordered aircraft with additional setting. I seem to remember 15° and 40°, but the above was the 'standard'.

Why the staggered retraction?

On takeoff, it's all about changing the wing from a low-speed wind to a high-speed wing. Normal takeoff flap setting for the -100/200 was 10°. I think it's 20° for the -400 but I never flew them. As you retract the flaps (both leading and trailing edge), the minimum safe airspeed increases approximately 20 knots for each decrease in flap extension. That gave you a safe bank angle of 15°. Add 10 more knots and you could, as I remember, bank 30°.
So, in the video the airplane is accelerating, and flaps are being retracted in stages as it reaches the minimum safe airspeed for that stage.
I'm not knowledgeable enough to authoritatively explain why, if you want to change part of a swept wing to a lower-lift wing while leaving part of it as a higher-lift wing, you need to make the change to lower-lift from outboard to inboard, but I do know, at least I think, that you must avoid the possibility of the outboard wing stalling while the inboard is still flying. You need to retain outboard aileron control as long as possible. Thus, retract the outboard leading edge devices before the inboard devices.

Does each leading-edge flap configuration have its own unique setting on the pilot's flap-control lever?

No, the leading edge flaps do not have a separate selection. They're tied to the trailing edge flap setting. Note that flaps 1° doesn't actually put any trailing edge flaps down (although I seem to remember it took them out of the retract position and moved them backwards, but I may be wrong), but extends only two of the four leading edge flap sections.

On approach, are the leading-edge flaps extended in a reverse staggered order?

Yes. On approach you're slowing, and you must not slow below the minimum safe speed for your flap setting. Those speeds will be lower than the respective takeoff speeds because you're lighter. I just looked up the fuel for a -400 that I have on my weight and balance web page, and it shows 359415 max fuel. Thus it's not unreasonable that you might be as much as 340000 lbs lighter than at takeoff and the minimum safe airspeed for a given flap setting could conceivably be in the range of 40 to 50 knots slower than at takeoff.
For those interested in more detail about the -400 leading edge devices, go to this incident report detailing an un-commanded retraction of the leading edge devices inboard of the engines. The single paragraph that most caught my attention is:

The serious incident involved the un-commanded retraction of the automatic
  Group ‘A’ leading edge flaps on rotation for a period of about 23 seconds.
  Subsequent to the initiation of the retraction of the Group ‘A’ leading edge flaps, the aircrew was faced with unexpected stall warnings. The pilot flying was able to prevent the aircraft from stalling, with support from the other crew members and to keep the aircraft flying until the leading edge flaps re-extended and normal performance capability returned.

I would expect the retraction to be sequenced, so the hydraulic system does not have to move them all at once. Would make sense to retract them outboard-most first.

Shouldn't the answer get into tip-stalling (and washout) with swept wings?

@PeterKämpf I can't remember if the -100/200 leading edge devices were hydraulically or pneumatically powered, but the incident report I added to my answer says the -400 uses pneumatic power with electrical backup. In any event, at least on the -100/200, whatever system was used would best be able to move them all at once because there was nothing to prevent the pilot from continuing to accelerate with flaps 5 until he had reached the flaps up speed, thus skipping the flaps 1 speed retraction and going directly from flaps 5 to flaps up.

@amI I added paragraph, but I'm really not knowledgeable when it comes to aerodynamics. Perhaps you'd be willing to answer it in more correct detail. And please do let me know if the added paragraph is incorrect.

It looks accurate, and good enough to clue the reader.

Flaps, Krueger flaps, and slats are all ways of modifying the wing's airfoil to effectively increase its camber.

Stalling happens when the air-flow separates. That is, rather than continuing to flow along the surface of the wing, it starts to simply flow away from the wing. It typically happens at high angles of attack.

When the wing's camber is increased, it's effectively operating at a higher angle of attack, even without moving the rest of the plane to a nose-up attitude.
As @Terry pointed out, you want to ensure that if any stalling happens, it happens toward the root of the wings, not the tip. To assure that, we typically want to ensure that the tips of the wings operate at a lower angle of attack than the roots of the wings. There are a number of ways to accomplish that.
One way to do this is to introduce washout--that is, the wing is twisted1, so the tips are always at a slightly lower angle of attack than the inboard parts. The ailerons also have an effect here though: when the aileron is moved downward, that also increases the angle of attack over that section of the wing. To be fully effective, you need enough washout that the angle of attack with the aileron at its extreme downward position is still lower than the angle of attack at the root of the wing. Otherwise, using the ailerons can stall the tip of the wing--and it's the wing you're trying to raise that ends up stalling, which reduces its lift substantially (more than normal aileron deflection would) and increases its drag. As a result, the plane turns the opposite of the direction you were attempting to turn it.
Washout helps prevent this, but twisting the wing decreases the wing's efficiency all the time, even though most of the time there's no real chance of stalling, so you're losing a lot of efficiency in return for safety you only need a little bit of the time.
Deploying flaps/slats primarily toward the root of the wing increases the camber and effective angle of attack at the root of the wing without affecting the camber/angle of attack at the tip. When the aircraft is cruising, the flaps/slats are retracted, so the wing can fly (reasonably) optimally. Tip stalls are dangerous enough that in most cases, you'll still have a little washout, but substantially less than you'd need if your also had flaps and/or slats operating at the tips of the wings.

1. Note that twisting the wing is one obvious way to introduce washout, but there are also other ways. For example, theoretically optimal wing loading happens with an elliptical planform. If the tips of the wings have a chord larger than an elliptical planform would, that will normally decrease wing loading, and with it the effective angle of attack.

Off the the top of my head.. not all parts of the wing generate the same amount of lift and the staggering could be a way of balancing .. equalizing the forces on the wing.
I believe the area nearer the wing tips have a higher lift to drag ration (ie high lift.. low drag) and will generate lift earlier than the rest of the wing. As a result, the requirement for flaps is reduced at a much lower airspeed compared to the thicker (higher drag) part of the wing.
Anilv


text:
What is this unusual leading edge flap on a B737-200?
On this video of Air North's gravel runway-equipped B737-200, there's what appears to be a small leading-edge flap (Krueger flap?) inboard of the engine.  Refer to the screen shot below.  What is this?

Addendum
Notice in the video (and screen shot) that this device shows itself while no other flaps are extended.

Hard to say. Looks more like an access panel hanging down during servicing than a Kruger flap, which will be hinged at the front of the LE.

A closer view of a similar LE flap. I don't understand why the first answer by @TomMcW was downvoted, it seems to be the good one.

These are two leading edge Krueger flap surfaces at each side of the wing, you can see details about them according IPC and AMM documents I found over the internet:

Regarding the Addendum, and why it seems that there is no other flaps deployed, I found a similar topic at Airliner.net Forum. As it is mentioned there, when is no hydraulic pressure, they can be extended by their weight:

...it has to do with the shut off valves and the actuators relaxing with no hydraulic pressure on them.

and also:

...Pressurize the system and they all move to their present commanded position

and another sidenote:

...Someone pulled that Krueger Flap to the fully extended position. When they droop because of no pressure on the system there in the mid positions.

and as final note, this issue is also mentioned in the AMM:

We have removed the images in your answer due to a DMCA complaint from Boeing. I realize this breaks your answer a lot. Please consider updating it in a way that makes it useful again without using copyrighted images.

@animuson thanks for updating me. I found those documents on the internet. Now, I even can't see what images I had posted, and unfortunately, I don't remember them, to repost similar images.

The 737-200 does have Krueger flaps inboard of the engines as seen in this photo

source
It’s hard to really tell, but it looks like the outboard half of the flap is not entirely retracted.

Curious, where did you get that image? Is there a higher-res version available online?

Oh shoot! Did I forget to put a citation? I think it was on the site B737.com. I'll see if it's still in my browser history

@pr1268 There. I added a citation link


text:
Is c.g rear or fore of wing leading edge?
This video states that the c.g of most non tapered airplane is 25%-30% of wing chord REAR of the leading edge.The aerodynamic center(a.c) of most wing is 25% rear of the leading edge ....Does this mean the a.c can coincide with the c.g in a tailplane?

Your title and body ask different questions. Also it’s recommended to use full words (not abbreviations) at least once.

For longitudinal stability in a conventional wing-and-tail layout, the CG needs to be ahead of the wing's AC.  This causes a nose-down moment that is countered by downward lift at the stabilizer, so that the stabilizer will raise the nose as speed builds up.  Thus, if the aircraft is disturbed into a dive, the nose will automatically rise as speed builds; conversely, if disturbed nose up, as speed falls off the nose will tend to drop.
Aircraft with active control (especially computer control), as well as some alternative layouts (such as a classic canard) can alter this conventional stability setup -- for instance, in a canard, the stabilizer at the nose lifts, and the CG will be ahead of the leading edge of the wing in most cases.

Actually, for stability the CG needs to be ahead of the overall neutral point, which is a bit behind the wing aerodynamic centre. But since stability increases as the CG moves forward, usually it ends up ahead of wing AC.

It's simpler: CG needs to be ahead of the whole aircraft AC (aka neutral point), regardless of the aerodynamic configuration (canard or classic etc.) Whether the tail produces negative lift is immaterial; it is easily possible to make a stable classic design with positive tail lift: you just need to ensure that the tail flies at a lower lift coefficient than the wing. Also, longitudinal stability per se is the short-period AoA stability (the airplane tends to restore AoA when it's disturbed); speed stability is a derived concept.

@Zeus I agree, but IMO that's more answer than this question asked for.

Yes, the centre of gravity can coincide with the aerodynamic centre of the main wing, particularly because that point is rather unremarkable. For stability, the centre of gravity needs to be ahead of the neutral point, but that is the overall aerodynamic centre if both main wing and horizontal stabilizer are flying at the same coefficient of lift, which means it is behind that of the main wing itself.
Aft limit of 30% of mean aerodynamic chord seems pretty typical, though IIRC the forward limit is usually further forward than 25%.

The aerodynamic center(a.c) of most wing is 25% rear of the leading edge

Yes, but keep in mind that this is only a typical value. In particular supercritical airfoils tend to have it quite a bit further aft. But you rarely see those on straight-wing aircraft; usually on swept-wing ones.

According to this https://en.m.wikipedia.org/wiki/Longitudinal_static_stability  , doesnt this mean that an airplane with c.g at the aerodynamic centre will have neutral stability?

@DavidTeahay, that wikipedia article uses the term “aerodynamic centre” to mean the “neutral point”. Unfortunately the term is used to mean other things in other contexts (especially aerodynamic centre of the airfoil vs. aerodynamic centre of the aircraft); the term “neutral point” is clearer.


text:
What is the purpose of this span wise "false spar" on Affordaplane wings?

This is the wing of an Affordaplane Ultralight,what is the use of those spanwise foam boards between the foam ribs,I know it's not for aesthetics.

If you push up on the rib from below, it will eventually start to deform sideways, along its least stiff axis, and collapse.  The pieces in between them are just braces that prevent sideways deformation, and making the rib stronger.


text:
To determine wing loading, do you need to subtract the weight of the wing from the gross weight?
Is it true that I must subtract the weight of my ultralight wing from the gross weight of my airplane inorder to determine the wing loading of my ultralight airplane? If yes,why is this so?
I read a pdf sometime ago stating that the weight of the wing is negligible when determining the wing loading.

No, you don't subtract any weight from gross when determining wing loading.
In airliners wings can be 40% of empty weight which is often about half of gross, so it would make a 20% difference in the loading calculation.

No.
The wing weight may or may not be negligible (usually not), but "wing loading", as a term, is simply defined with respect to the total weight. Wings have to carry all the weight, including their own, anyway.
Wing loading is a simple integral figure for the whole aircraft, and we use whole weight and whole lift for it.
Wing weight matters when we calculate its bending moment, but then we don't use the integral wing loading figure: we need the actual spanwise lift (and weight) distribution.


text:
What is the function of a sweep cable?
This is Goat3 ultralight with sweep cables,what is the fuction of these sweep cables?

To keep the wings straight.
If you remove them there is very little to prevent the wing from rotating around the vertical axis.
All the other cables are attached to the king post or the seat. This cannot provide the leverage needed to counteract the wing yawing relative to the seat.

Okay but I've observed that majority of the ultralights I've seen (images) don't actually have sweep cables,so how do they keep their wings "straight ".

The torque needs to be counteracted somewhere, It is possible to do it entirely internal to the wing mount, or using the chord length for leverage. Which one to pick depends on design decisions.


text:
At what angle of deflections do rudders become inefficient?
This article (picture) says rudders for ships can deflect 35° max,so at what aoa does an airplane rudder lose its efficiency,at what " angle of deflection " does it start to stall ?

Remember - deflection is NOT the same Angle of Attack. Angle Attack is the airfoil vs the relative airflow which may not always be coming from the front.

Ok....true....will correct that @Dan

Hydrodynamics are my bread and butter... remember, a ships rudder has the prop right in front of it. Any more than 35 degrees and you are probably going to rip the rudder off with the prop wash/rudder forces. There are some other things to think about but that is the major one that you don't have more deflection is just that, keeping the rudder attached.

Related: How does flight control surface authority change with AOA?

I think this comment about ships is about really specific types of ships. I've sailed dinghies using more than 35° of rudder to turn.

The only time you should be using full rudder is when doing a side slip - such as turning the yoke for a left turn but pressing right rudder, so you are descending and looking to your left as the plane goes forward tracking down the runway direction.  Normal coordinated turns, you would not use anywhere near that much rudder while turning.
I just found a NASA Test Report on my plane, a Cessna Cardinal, that lists control deflection amounts as a reference for a light general aviation airplane:
Angle of attack,           -4° - +20° ± 0.5°
Sideslip,                 -15° - +15° ± 0.5°
Pitch angle,              -15° - +15° ± 0.5°
Roll angle,               -75° - +75° ± 0.5°
Rudder deflection,        -24° - +24° ± 0.5°
Stabilator deflection,    -20° - + 5° ± 0.5°
Right aileron deflection, -20° - +20° ± 0.5°
Left aileron deflection,  -20° - +20° ± 0.5°

Yes!....finally a nail in the head!.... Thanks bro!

This does not answer the question. This table quoted here is grossly missrepresented; the original document list this table as the parameters that the test instrumentation measures and records. It does not present actual control deflection muchless effectiveness at any specific deflection.

Most airplanes seem to be around 23-25 degrees.  I would imagine water rudders can go to 35 because of the much lower Reynolds number of a rudder in water (much higher fluid viscosity/lower velocity).

The aerobatic Extra 300 seems to do up to 30° each direction on the ailerons - see https://www.extraaircraft.com/docs/service/POH%20EA%20300%20CUS%202006-09-20.pdf

OP is not going to be doing aerobatics in a homebuilt ultralight tho.

True!.......love my life

What does this editorial comment mean? "Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted."  The data is right from the NASA report, which is linked to in the answer.